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Analysis of Multipath Interference for SAR Countermeasures Trials

This document presents a model developed to study the impact of multipath interference on jammer performance during Electronic Counter-Measures (ECM) trials against Synthetic Aperture Radars (SARs). The work was motivated by the low grazing angle geometry of the NATO SCI-066 AhrCarSAR trial, where the SAR was driven on a long, high bridge while imaging the valley below. A simple model of specular reflection was used and analytical formulas were developed to make predictions. It was found that the geometry of the AhrCarSAR trial was subject to multipath interference, as are some airborne geometries as well. The analysis of the model showed that the interference of direct and reflected-path pulses results in a chirp with a power modulation determined mainly by the reflection coefficient of the surface near the jammer. For horizontal polarizations, the reflection coefficient approaches 1 and multipath effects can be important. For veritcal polarizations however, the reflection coefficient can be significantly less than 1 and multipath effects are weak. Since a vertical polarization is used for the AhrCarSAR trial, it was predicted that the performance of the jammer would not be significantly affected by multipath interference.